Communications systems arose as mechanisms to connect individual telephones to each other. Users dialed phone numbers and were connected through a network to other telephones associated with those numbers. This is still the primary mode of interaction in telecommunication systems in both public and private networks. For a business, this form of telecommunications function is typically provided by a Private Branch Exchange (PBX) on company premises or by a Centrex system offered by a telephone company. In a PBX or Centrex system the system is aware of the “state” of telephone lines rather than the state of the individuals (referred to as “users” herein) and is typically limited to the busy and idle line states.
Other types of telecommunications systems do exist but are specialized in their applications. Of particular importance is the Automatic Call Distributor (ACD) which is used, for example, in an airline reservation bureau to keep a caller on hold until an appropriate agent is ready to take the call. In these systems, a person dialing in is one type of user, and the agents are another. As opposed to the PBX, these systems route calls with respect to identified people (i.e., the next available agent), and not just telephone lines and numbers, and they do so using a concept of state that reflects what the agent is doing, not just whether the agent's phone is busy. However, the states are specific to the call center environment, and the communications role is restricted to dequeuing based on target user state. While ACDs cost significantly more than PBXs to install, the efficiency gains provided by state-dependent routing justify the increased expense.
Another form of communication often packaged with an ACD is a callback system. With these systems a party can request a callback according to a schedule. In this case a communication is established according to the schedule and availability of agents for callback work. Callback requests can be placed by phone or other means including the internet.
Instant messaging systems offered by America On-Line and Microsoft are a different type of communication system that has been developed recently and used widely. With an Instant Messaging client application, a user can set his own state to one of a discrete number of values (For Microsoft: On-line, Busy, Be Right Back, Away, On the Phone, Out to Lunch, Appear Off-line), and these values will be delivered to others who have signed up with the system to receive them. The system also enables the user to control who is allowed to observe him. The state information is used primarily by individuals who want to know if their friends and colleagues are on-line, so that they can be contacted. There is currently an international standards activity in the Internet Engineering Task Force (IETF) to produce uniform protocols for these state delivery systems.